Showing papers on "Dissolution published in 1990"

Growth of native oxide on a silicon surface

Abstract: The control factors controlling the growth of native silicon oxide on silicon (Si) surfaces have been identified. The coexistence of oxygen and water or moisture is required for growth of native oxide both in air and in ultrapure water at room temperature. Layer‐by‐layer growth of native oxide films occurs on Si surfaces exposed to air. Growth of native oxides on n‐Si in ultrapure water is described by a parabolic law, while the native oxide film thickness on n +‐Si in ultrapure water saturates at 10 A. The native oxide growth on n‐Si in ultrapure water is continuously accompanied by a dissolution of Si into the water and degrades the atomic flatness at the oxide‐Si interface, producing a rough oxide surface. A dissolution of Si into the water has not been observed for the Si wafer having surface covered by the native oxide grown in air. Native oxides grown in air and in ultrapure de‐ionized water have been demonstrated experimentally to exhibit remarkable differences such as contact angles of ultrapure waterdrops and chemical binding energy. These chemical bond structures for native oxide filmsgrown in air and in ultrapure water are also discussed.

Nonaqueous Electrochemistry of Magnesium Applications to Energy Storage

Abstract: Research leading to the construction of an ambient temperature rechargeable magnesium battery based on organic electrolytes and positive electrodes capable of reversible intercalation of Mg+2 ions is discussed. The number of combinations of solvent, solute, and intercalation cathode which give reasonable battery performance is much more limited for Mg than for alkali metals. The only electrolytes which allowed Mg dissolution and deposition were solutions of organomagnesium compounds in ethers or tertiary amines; many of these were unstable in the presence of transition metal oxides or sulfides which were found to function acceptably as intercalation electrodes. Possible directions for future research which could solve these problems are discussed, as well as theoretical aspects of magnesium compound behavior in nonaqueous solvents.

Geochemistry of hydrothermal fluids from Axial Seamount hydrothermal emissions study vent field, Juan de Fuca Ridge: Subseafloor boiling and subsequent fluid‐rock interaction

Abstract: Hydrothermal fluids collected from the ASHES vent field in 1986, 1987, and 1988 exhibit a very wide range of chemical composition over a small area (∼60 m in diameter). Compositions range from a 300°C, gas-enriched (285 mmol/kg CO2), low-chlorinity (∼33% of seawater) fluid to a 328°C, relatively gas-depleted (50 mmol/kg CO2), high-chlorinity (∼116% of seawater) fluid. The entire range of measured compositions at ASHES is best explained by a single hydrothermal fluid undergoing phase separation while rising through the ocean crust, followed by partial segregation of the vapor and brine phases. Other mechanisms proposed to produce chlorinity variations in hydrothermal fluids (precipitation/dissolution of a chloride-bearing mineral or crustal hydration) cannot produce the covariation of chlorinity and gas content observed at ASHES. There is good agreement of the measured fluid compositions with compositions generated by a simple model of phase separation, in which gases are partitioned according to Henry's law and all salt remains in the liquid phase. Significant enrichments in silica, lithium and boron in the low-chlorinity fluids over levels predicted by the model are attributed to fluid-rock interaction in the upflow zone. Depletions in iron and calcium suggest that these elements have been removed by iron-sulfide and anhydrite precipitation at some time in the history of the low-chlorinity fluids. The distribution of low- and high-chlorinity venting is consistent with mechanisms of phase segregation based on differential buoyancy or relative permeability. The relatively shallow depth of the seafloor (1540 m) and the observed chemistry of ASHES fluids are consistent with phase separation in the sub-critical or near-critical region.

Theory of Steady‐State Passive Films

Abstract: A theory is developed for the steady-state properties of passive films that form on metals and alloys in aqueous environments. This theory is based on the point defect model developed earlier, and predicts that they steady-state thickness of the barrier film and the log of the steady-state current will vary linearly with applied voltage. These relationships may be used to estimate empirical parameters that describe the dependencies of the potential drop across the barrier film/environment interface on the applied voltage and pH and to estimate kinetic parameters for dissolution of the film. If dissolution at the film-solution interface occurs very slowly, the primary passive film is envisaged to consist of a rigid oxide sublattice that transmit cations from the metal to a gel-like, precipitated upper layer. If dissolution at the barrier film/environment interface occurs rapidly, then a steady-state thickness is achieved by a balance between the rate of dissolution of the film at the film-solution interface and the rate of growth of the film into the underlying metal phase. Due to the outward movement of oxygen vacancies (i.e., inward movement of oxygen ions) through the barrier layer. The model is found to account for many experimental data that have beenmore » published in the literature on the quasi steady-state properties of passive films.« less

Coordination chemistry of weathering: Kinetics of the surface-controlled dissolution of oxide minerals

Abstract: Chemical weathering processes, essentially caused by the interaction of water and the atmosphere with the Earth's crust, transform primary minerals into solutes and clays and, eventually, into sedimentary rocks; these processes participate in controlling the global hydrogeochemical cycles of many elements. Many mineral dissolution processes are controlled by a chemical mechanism at the solid-water interface. The reaction-controlling steps can be interpreted in terms of a surface coordination model. The tendency of a mineral to dissolve is influenced by the interaction of solutes—H+, OH−, ligands, and metal ions—with its surface. The surface reactivity is shown to depend on the surface species and their structural identity; specifically, the dependence of dissolution rates on pH and on dissolved ligand concentrations can be explained in terms of surface protonation (and deprotonation) and of ligand surface complexes. A general rate law for the dissolution of minerals is derived by considering, in addition to the surface coordination chemistry, established models of lattice statistics and activated complex theory.

Preparation of liposome and lipid complex compositions

Abstract: Liposome and lipidic particle formulations of compounds are prepared by dissolving in a solution of liposome-forming lipids in an aprotic solvent such as DMSO, optionally containing a lipid-solubilizing amount of a lower alkanol, and either injecting the resulting solution into an aqueous solution, or the aqueous solution into the resulting solution. The resulting liposome or lipidic particle suspension may then be dialyzed or otherwise concentrated. This method is particularly useful for compounds which are poorly-soluble in aqueous solution, but is generally useful for any compound or combination of compounds which can be dissolved in the aprotic solvent or aprotic solvent/lower alkanol mixture.

Solubility behavior of organic compounds

Abstract: Solubility, Intermolecular Forces, and Thermodynamics Activities of Solutes, Selection of Standard State and Henry's Law Constants Specific Interactions in Solubility Phenomena Influence of Complexation, Especially Hydrogen Bonding, on Solubility in Organic Solvents: Prediction of Association Constants Structure of Solvents and Effects of Self-Association on Solubility Group Contributions in Prediction Solubility in and Partitioning into Water Ion Pairs and Solubility Behavior Phase-Solubility Analysis and Complexation in Aqueous Solution Dissolution Rates of Solids Afterword References Author Index Subject Index.

The application of electrochemical impedance techniques to aluminium corrosion in acidic chloride solution

Abstract: Electrochemical impedance techniques are applied to the study of aluminium corrosion in hydrochloric acid solution in the passive region. Impedance spectra are obtained in stationary solution and in solution moving due to electrode rotation. The form of the spectra shows the importance of multistep dissolution, ionic migration through the oxide layer, relaxation effects and the influence of chloride ion.

The Effect of Temperature on the Acidic Dissolution of Steel in the Presence of Inhibitors

Abstract: The corrosion of mild steel in sulfuric acid containing 4 × 10−4 M of either D-galactose thiosemicarbazone, 1-phenyl thiosemicarbazide, or 4-phenyl thiosemicarbazide, has been studied over...

Rock magnetic evidence for the dissolution and authigenic growth of magnetic minerals within anoxic marine sediments of the California continental borderland

Abstract: The rock magnetic properties of marine sediments from the California continental borderland (San Pedro, Santa Catalina, and San Nicolas basins) have been studied in order to quantitatively assess the effects of sediment diagenesis on magnetic minerals. Previous studies have noted that the natural remanent magnetization in these sediments, primarily carried by detrital magnetite, decays to 10% or less of its surface value soon after deposition. This decrease is caused by magnetite dissolution related to sediment diagenesis and is unrelated to paleoclimatic variations or changes in the regional influx of detrital magnetic material. Detailed rock magnetic measurements show that shifts to softer remanent coercivity and differences in the rate and degree of magnetic intensity loss with depth can be related to the dissolution process. The shift to softer remanent coercivity is related to a coarsening of the magnetic mineral grain sizes with depth due to preferential dissolution of the finest-grained magnetic material. The intensity decreases, which are linearly proportional to magnetite concentration decreases, indicate that dissolution occurs with rate constants ranging from 0.3 to 1.6 kyr−1. The rate constants, sulfide concentrations, and magnetite grain size estimates from the borderland are consistant with previous studies of magnetite dissolution. Our results demonstrate the importance of both sulfide and magnetite surface area in the dissolution process. Anomalous peaks in viscous remanence within the sediments suggest the authigenic growth of greigite and its subsequent transformation to pyrite.

Diagenesis in anoxic sediments from the California continental borderland and its influence on iron, sulfur, and magnetite behavior

Abstract: Solid phase and pore water profiles of compounds containing iron and sulfur have been determined by wet chemical, magnetic, and X-ray diffraction techniques in three California continental borderland basins. The observed profiles have been fit by simple reaction-diffusion models in order to determine reaction rates and constrain budgets for iron and sulfur. More than 95% of the solid phase reduced sulfur is pyrite, and down core profiles are well fit by a model in which net sulfate reduction rates decrease exponentially with depth. Net sulfate reduction rates determined from models fit to solid phase reduced sulfur measurements and pore water sulfate profiles yield results that are consistent. Depth integrated sulfate reduction rates for modern sediments in San Pedro, Santa Catalina, and San Nicolas Basins are, 11.4, 6.3, and 6.3–8.8 (μmol cm-2yr-1), respectively. Measurements of solid phase iron species indicate that surficial sediments are enriched in easily-reducible ferric oxyhydroxides. The enrichment is maintained by a combination of oxidation of Fe2+ diffusing upward from underlying anoxic sediments, as well as input of fresh sediment enriched in ferric oxyhydroxides. The three primary sources for iron converted to pyrite and the sequence in which they are utilized are: ferric oxyhydroxides, magnetite and other crystalline oxides, and “exchangeable” iron in phyllosilicates. The majority (50–80%) of the iron converted to pyrite is from the silicates, and budgetary calculations indicate the amount of iron released from San Pedro basin silicates agrees within 35% with the amount of magnesium removed from pore water to solid phases. Fe2+ is enriched in near-surface pore waters because rates of dissolved iron production by oxyhydroxide reduction exceed rates of sulfate reduction and pyrite formation. At depth, pore waters are sulfidic because rates of sulfate reduction exceed rates of iron release from silicates. Sulfide produced at depth diffuses upward until it reaches sediments with available iron, causing a step-like increase in solid phase sulfur concentration. Over 90% of the magnetite present in surficial sediments is dissolved at depth due to reaction with H2S. A model is developed to predict the depth at which magnetite dissolution should occur, based on sulfate reduction rates and the flux of ferric oxyhydroxides. The results of this model predict the onset of dissolution at depths of 5–40 cm in different basins and agree well with the observed depths of magnetite dissolution.

Polyethyelene Glycols and Drug Release

Abstract: The dissolution rates of several drugs may be increased by incorporation into solid polyethylene glycols1. These dispersions are usually manufactured by heating a physical mixture of the drug and polymer to the fluid state and subsequently cooling to room temperature.The physical structure of both the drug and the polyethylene glycol will be discussed, as these factors may affect the rate of drug release from the dispersions2,3. The solid state properties of both components have traditionally been studied by X-ray diffraction and/or by differential scanning calorimetry (DSC). The latter technique has facilitated the use of phase diagrams in the investigation of the melting properties of the dispersions, these usually indicating the presence of eutectics, monotectics, solid solutions or glasses. The application of a further technique, dielectric spectroscopy, in the study of molten and solid dispersions will be described.The mechanisms by which drug dissolution rate may be enhanced will be describe...

Corrosion Mechanisms for Iron and Low Alloy Steels Exposed to the Atmosphere

Abstract: Despite extensive study over the years, the chemical processes involved in the atmospheric corrosion of iron and its alloys remain poorly understood. Most conceptual studies have ignored the chemical influence of the trace anions (Cl−,, , , etc.) present in the atmosphere and in precipitation. This review, presented from the perspective of atmospheric chemistry and mineralogy, provides an analysis of rust layer formation, evolution, morphology, and composition, together with information on iron‐containing minerals and other crystalline structures that are likely to be present. The chemical reactions involved in the formation of these constituents during the corrosion process are then presented. The reactions are not spatially homogeneous, but favor pits, voids, and crevices in the metal surface. It is demonstrated that (i) the pH of the moisture on the surface is crucial to the corrosion process, since it controls the dissolution of the passive oxyhydroxide surface; the pH is largely controlled by atmospheric and dissolved in the moisture or by fog or rain deposited on the surface; (ii) the extant data suggest that the rate of iron oxyhydroxide formation is slow; hence, the presence of reactive anions generally results in their blending into mixed hydroxy‐anion products; (iii) the interactive chemistry of readily available hydrogen peroxide and bisulfite ion in the aqueous surface film can either enhance or impede the rate of corrosion; (iv) photon‐driven reactions can promote the corrosion of iron and its alloys. This analysis unifies the analytical information, as well as the data on kinetic processes, and provides the basis for a full understanding of the atmospheric corrosion of iron and low alloy steels.

Hydrothermal crystallization kinetics of m-ZrO2 and t-ZrO2

Abstract: The isothermal nucleation and crystallization kinetics of hydrothermally prepared monoclinic and tetragonal ZrO2 have been determined at various pH conditions. It is shown that monoclinic ZrO2 precipitates at low pH whereas at high pH tetragonal ZrO2 crystallizes from an amorphous zirconium (hydrous) oxide, Zr(OH)xOy, precursor. At intermediate pH conditions mixtures of the polymorphs are formed suggestive of kinetically competing particle formation mechanisms. The data are explained by the proposed existence of three controlling regimes for the formation of crystalline ZrO2: dissolution/precipitation at low pH, a solubility controlled regime at intermediate pH values, and a gel structure controlled regime at high pH. Apparent activation energies for the nucleation and crystallization of monoclinic and tetragonal ZrO2 formed under hydrothermal conditions are presented.

Interaction of 18Cr-10Ni stainless steel with liquid aluminium

Abstract: The dissolution of an 18Cr-10Ni stainless steel in liquid aluminium at 700 to 850 ° C was found by the rotating disc technique to be non-selective and diffusion controlled Experimentally determined values of the parameters characterizing the dissolution run are presented In the case of saturated aluminium melts two intermetallic layers were found to form between the steel and the melt material at 700 °C The compact layer adjacent to the steel surface is probably a solid solution based upon the Fe2Al5 compound Its thickness,x, tends with increasing time to the limiting valuex max = 10μm The porous layer adjacent to the melt material is probably a solid solution based upon the FeAl3 compound After a certain period of non-linear growth its thickness,Y, increases with time,t, according to the linear law:Y = 1 × 10−8 t + 6 × 10−6 m The time dependence of the total thickness of both layers is well described in terms of the “paralinear” kinetics In the case of undersaturated aluminium melts the formation of a single-phase intermetallic layer, 3 to 11 μm thick, was observed at 700 ° C for 100 to 600 sec The steel-to-aluminium transition joints with good mechanical properties were made by interaction of a solid steel material with liquid aluminium

The reductive dissolution of iron oxides by ascorbate

Abstract: There are four general pathways of dissolution of reducible metal oxides in acidic aqueous solution: proton-assisted (acid), ligand-promoted acid, reductive, and ligand-promoted reductive dissolution. The presence and reactivity toward the surface of protons, chelating ligands, and reductants dictate the mechanism(s) controlling the dissolution. For the massive reductive dissolution of magnetic by ascorbic acid, the experimental rate law R = k[HA−]12[H+] suggests the involvement of surface ≡FeIII A− complexes. Adsorption isotherms of ascorbic acid onto hematite at pH 3 and 25°C yield a Langmuir-type surface complexation constant Ks = (9.57 × 108 M−1). Slow dissolution follows with an empirical rate law R = kobs(≡FeIIIA). It is concluded that the formation and kinetic reactivity of surface complexes determine the rate of dissolution. Dehydroascorbic acid also dissolves magnetite, but at slower rates. Oxalate accelerates the reductive dissolution of hematite by ascorbate even though it competes with ascorbate for surface sites; enhanced detachment of ≡FeII surface species by oxalate complexation may be involved. Autoacceleration of the reductive dissolution by dissolved FeII-carboxylate complexes is observed in EDTA/ascorbic acid mixtures; the rate reaches a maximum at intermediate [EDTA] values, where synergistic effects between EDTA and FeII-EDTA complexes are important. Autoacceleration may also operate in oxalate solutions.